A. Field of the Invention
The present invention relates to the field of data compression and, more particularly, to systems and techniques for compressing digital motion video signals.
B. Related Art
Technological advances in digital transmission networks, digital storage media, Very Large Scale Integration devices, and digital processing of video and audio signals are converging to make the transmission and storage of digital video economical in a wide variety of applications. Because the storage and transmission of digital video signals is central to many applications, and because an uncompressed representation of a video signal requires a large amount of storage, the use of digital video compression techniques is vital to this advancing art. In this regard, several international standards for the compression of digital video signals have emerged over the past decade, with more currently under development. These standards apply to transmission and storage of compressed digital video in a variety of applications, including: video-telephony and teleconferencing; high quality digital television transmission on coaxial and fiber-optic networks as well as broadcast terrestrially and over direct broadcast satellites; and in interactive multimedia products on CD-ROM, Digital Audio Tape, and disk drives.
Several of these standards involve algorithms based on a common core of compression techniques, e.g., the CCITT (Consultative Committee on International Telegraphy and Telephony) Recommendation H.120, the CCITT Recommendation H.261, and the ISO/IEC MPEG-1 standard (ISO/IEC JTC1/SC2/WG11 MPEG 91/74 MPEG Video Report Draft, CD-11172, 1991) and and MPEG-2 Standard (ISO/IEC 13818-2, 1994).
The MPEG standards have been developed by the Moving Picture Experts Group (MPEG), part of a joint technical committee of the International Standards Organization (ISO) and the International Electrotechnical Commission (IEC). The MPEG committee has been developing standards for the multiplexed, compressed representation of video and associated audio signals. The standards specify the syntax of the compressed bit stream and the method of decoding, but leave considerable latitude for novelty and variety in the algorithm employed in the encoder.
The MPEG-1 standard was developed for use in compressing progressive video. A progressive video sequence is a sequence in which each frame represents a scene as it is viewed at a discrete time instance. By contrast, for interlaced video, a field--every other line on the screen--is captured periodically. For interlaced video, at alternating time instances the top and bottom field on a screen is refreshed. At any given time, data from two fields--a frame--can be seen.
The MPEG-2 standard, can be used to compress either interlaced video, progressive video, or a mixture of progressive and interlaced video: the encoder specifies whether each frame is progressive or interlaced.
By today's yardstick, compressing a video sequence into high quality MPEG bit stream is a computationally intensive task. The video compression process is sequential in nature, i.e., there are many temporal dependencies. These dependencies arise primarily from the need to do motion estimation and the need to stay within the specified bit-rate constraints, i.e., rate control. As a result of these dependencies, it is difficult to use multiple processors to parallelize (and hence speed up) the compression process.
One conventional method of performing MPEG-1 compliant video compression is to parallelize the compression at the macroblock level, i.e., each processor compresses a macroblock. While superior to pure sequential processing, this fine grain of parallelism leads to significant communication overheads among processors and between the processors and the shared memory.